Changes in body composition, including a decrease in bone and muscle mass and an increase in the proportion of fat occur with aging. Lower muscle mass, or the reduction of muscle mass, known as sarcopenia, is associated with lower strength and is thought to contribute to the development of functional limitations and disability in old age. Until the advent of dual-energy x-ray absorptiometry (DXA) scanning, it was difficult to reliably quantify lean mass in large population studies, so the issue of precise clinical definitions had not been paramount. Now that DXA scanning is widely available for osteoporosis screening, the ability to classify large numbers of persons as sarcopenic is feasible.[unreadable] Recent evidence suggests that both lower lean mass and higher body fat may be independently associated with poor function of older adults. Thus, studies are needed to address whether it is possible to intervene in older adults in order to preserve their lean mass and decrease fat mass. Exercise, or increased physical activity, may indeed achieve this goal. [unreadable] This study is funded through a cooperative agreement with the Geriatrics and Clinical Gerontology Program at NIA and the DXA research is an add-on substudy. Adding simple measures of body composition to the trial using DXA is an efficient way to address a novel aspect of how increased physical activity may induce favorable changes in body composition in older adults and further, may help determine appropriate interventions to improve body composition and function. All baseline and follow-up scans have been completed and are currently being analyzed. The results were presented at the Gerontological Society of America meeting and a manuscript is in preparation.